Regional Stratigraphy and Sedimentology of Phosphatic Event Beds and Associated Strata in the San Gregorio Formation (Upper Oligocene-Lower Miocene), Baja California Sur, Mexico

Pi‘s将结合地层学、沉积学和岩石学方法对富含磷酸盐的San Gregorio组(上渐新世-下中新统)进行调查，这是墨西哥南巴哈加利福尼亚的一个广泛且具有重要经济意义的单位。与墨西哥研究人员合作，我们的目标是澄清古地理和事件沉积在磷岩和相关地层起源中的作用。初步野外观测表明，圣格雷戈里奥组是事件分层的。我们的发现表明，许多磷酸盐沉积物的运输在成因上与火山喷发事件有关；此外，磷酸盐事件地层的沉积仅与短期的动物群生物扰动间隔有关。磷酸盐颗粒类型差异很大，但在个别层位内始终是单峰的；我们希望了解磷酸盐颗粒类型的系统地层和区域差异，并破译这些变化对异地磷矿源区演化的意义。对磷酸盐颗粒的岩石学研究，包括对抛光薄片的反向散射扫描电子显微镜的使用，将使观察到的磷酸盐颗粒类型的趋势与圣格雷戈里奥组内的区域沉积古地理相关联，并将进一步证明微生物和其他生物群在磷酸盐颗粒起源中的联系。

PI's will integrate stratigraphic, sedimentologic and petrologic approaches in an investigation of the phosphate-rich San Gregorio Formation (Upper Oligocene- Lower Miocene), a widespread and economically-important unit in Baja California Sur, Mexico. Working collaboratively with Mexican investigators, we aim to clarify the roles of paleogeography and event deposition in the origin of phosphorites and associated strata. Initial field observations indicate that the San Gregorio Formation is event stratified. Our findings suggest that transportation of many phosphatic sediments was genetically- linked to explosive volcanic episodes; furthermore, deposition of phosphatic event strata was exclusively associated with short- duration intervals of infaunal bioturbation. Phosphatic grain types vary widely, but are consistently unimodal within individual beds; we wish to understand the systematic stratigraphic and regional variability in phosphatic grain type and to decipher what these variations signify about the evolution of source areas for allochthonous phosphorites. Petrologic studies of phosphatic grains, including the employment of backscatter SEM on polished thin sections, will permit correlations between observed trends in phosphatic grain type and regional sedimentary paleogeography within the San Gregorio Formation and will further document the association of microbes and other biota in the origin of phosphatic grains.